Decoupling capacitors in semiconductor circuitry are essential components used to filter much of the noise that may be present between operating supplies such as power and ground. Some fabrication processes construct thin film decoupling capacitors on a silicon substrate by forming one electrode into the substrate itself, and then forming a second electrode out of an overlying conductive material with the two electrodes being separated by a dielectric material. In U.S. Pat. No. 5,304,506 to Porter et al., a further electrode is formed, overlying the second electrode and is electrically isolated therefrom by a dielectric material. The second and third electrodes form a second decoupling capacitor which can be coupled in series or in parallel depending on the choice of the circuit designer. A series connection will protect against one of the decoupling capacitors becoming shorted out by errant subsequent implants. If one capacitor shorts, the other will adequately decouple the noise. If coupled in parallel, the overall decoupling capacitance is increased.
The use of depletion mode decoupling capacitors is taught in U.S. Pat. No. 5,266,821 to Chern et al., and U.S. Pat. No. 5,032,892 to Chern et al. The capacitors are placed in open space which is not being used for other circuitry. However, as circuit densities increase, there is less and less such open space. There is still a need to increase the circuit density of integrated circuits, especially in dynamic random access memory (DRAM chips. The storage densities of such chips is growing at a phenomenal rate, and there is a great need to both utilize all the space available for memory cells, and to reduce the size of structures and line widths to provide more memory cells and circuitry in the same space. The need for capacitive decoupling of such circuitry actually increases in importance as the density increases. The same level of power supply voltage spikes and noise exist, and with decreased line widths, have an even more damaging effect.
There is a need for yet further increasing the flexibility and reliability of decoupling capacitors. Yet a further need exists to provide fully isolated capacitor nodes to eliminate the need for bias devices and further conductive paths.